In situ combustion of carbonaceous strata



Oct. 17, 1961 F. w. CRAWFORD ET AL IN SITU COMBUSTION OF CARBONACEOUS STRATA Filed March 21, 1958 PRODUCT l TO TANK PRODUCTION WELL 1: INJECTION WELL AIR SUPPLY 'IHEATER WELL PATTERN INVENTORS F.W. CRAWFORD H.W. PARKER ATTORNEYS ted States FrancisW. Crawford and Harry W. Parker, Bartlesville,

kla., assignors to Phillips Petroleum Company, a corporation of Delaware Filed Mar. 21, 1958, Ser. No. 722,885 12 Claims. (Cl. 166-11) This invention relates to a process for initiating combustion in carbonaceous strata and for producing hydrocarbon material therefrom.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum drives out and usually upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein followed by injection of air thru the ignition borehole and recovery of product hydrocarbons and combustion gas thru another borehole in the stratum is a direct air drive process for elfecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difiiculty and to permit the continued progress of the combustion zone thru the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the combustion zone from one or more surrounding boreholes.

In certain types of strata, particularly in tar sand formations, even when utilizing inverse air injection, there is difficulty in maintaining a high enough oxygen concentration in the heated area surrounding the ignition borehole to effect and continue self-sustaining combustion. The reason may be that the gas stream reaching the heated stratum is too fuel-rich from vaporization and/or cracking of the tar, or too much oxygen is lost by slow oxidation of the stratum, or vaporization of water in the formation dilutes the oxygen to a low concentration.

Another problem encountered when establishing in situ combustion in certain strata lies in the plugging of the formation in an area adjacent the heated zone by liquid hydrocarbons released from the stratum. As heat is applied to the carbonaceous stratum around an ignition borehole solid and semi-solid hydrocarbon material is rendered fluid and the supply of oxygen is inadequate to burn this material and/or the combustion is hindered by inadequate mixing of the fuel and oxygen. The liquid 70 adequate supply of oxygen in the heated stratum. A

further object is to provide a method for igniting a caratent O Patented Oct. 17, 1961 1 ice bonaceous stratum which avoids plugging of the stratum with liquid-hydrocarbons released during the heating step. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the process of the invention comprises simultaneously heating a section of carbonaceous stratum around an ignition borehole from a heat source therein and alternately passing air or other Og-containing gas from the borehole into the heated stratum and thru the heated stratum into the borehole, and continuing the heating and flow of air in alternate directions until ignition is accomplished. The heating is effected by means of a gas-fired heater, an electric heater, or other heat source, such as glowing charcoal, positioned in the ignition borehole adjacent the stratum to be ignited.

The flow of air or other combustion supporting gas alternately in and out of the stratum may be effected in either of two ways. Air may be alternately injected thru the ignition borehole and thru one or more injection boreholes in the stratum spaced a short distance therefrom such as to permit air to flow from the injection borehole to the ignition borehole and vice versa. In this type of operation, the gas is alternately vented from the borehole opposite the borehole thru which air is being injected. In other words when air is being injected into the stratum thru the ignition borehole, gas is being vented from the air injection boreholes, and vice versa.

Another embodiment of theinvention comprises pressurizing the ignition boreholewith air and, as heating of the stratum fiuidizes hydrocarbons therein, alternately increasing and decreasing the air pressure in the ignition borehole. This has the effect of forcing air into the stratum and producing iluid hydrocarbons thru the well tubing and allowing air to be forced back into the borehole on reduction of pressure, which pushes fluidized hydrocarbons intothe ignition borehole. During this type of pressurizing the pressure can be maintained sufficiently high at all times to force liquid hydrocarbons collecting in the bottom of the borehole thru the well tubing.

In many applications of the process to various strata of the classes tar sands, oil sands, shales, porous coal veins, etc., the change in direction of ilow of air must be made at very short intervals so as to avoid plugging of the formation. In tight strata or strata containing a high concentration of heavy hydrocarbon materials, the maximum period of flow of air in either direction, particularly when the stratum is hot, is not over about three minutes and in many applications the maximum period is two minutes or less. By watching the pressure gauge on the injection air line into the borehole thru which air is being injected, the time for reversing the flow of air is indicated by a substantial pressure increase when the air supply is constant. This pressure increase precedes plugging of the stratum and indicates the need of reversing the flow of air so as to avoid plugging. Even in applications of the process to strata not prone to plugging, it is well to reverse the flow of air at frequent short intervals in order to improve the mixing of 0 with the fuel in the formation and to provide a fresh supply of O in the heated section of the stratum. This frequent alternating of flow of air is also applicable in pressurizing techniques wherein the pressure is alternately increased and decreased within the ignition borehole. A technique which is particularly advantageous comprises flowing the air in one direction a longer period than in the opposite direction in the stratum, e.g., for two minutes into the stratum and then for three minutes into the ignition borehole from the stratum.

A preferred technique comprises alternately injecting for exhaust purposes.

air into the stratum from an ignition borehole in which heat is being applied to the stratum and cutting off this flow of air and injecting air thru a series of injection boreholes surrounding the ignition borehole and spaced therefrom a distance of not more than 'about '5 feet so that injected air has time to ass completely 'thru the section of stratum between the ignition boreholes and the surroundin -boreholes. A spacing of 18 inches between-me ignition bore hole and the injection boreholes has been found effective in a tar sand. With seen a spacin the period required for heating the stratum completely th ru to the injection boreholes is relatively short and the hot stratum is readily accessible to the air at normal oxygen concentration and ignition of the stratum when the temperature-hasbeen "brought to the ignition point is :effected readily andwit'hont difiiculty.

Another embodiment of the invention comprises heating a permeable carbonaceous stratum by suitable heating means placed in the borehole adjacent the stratum until the rock and -oil are heated at least tofignition tempera: tures in-the range of 400 to 800 F., depending-upon the characteristics of thehydrocarbon material in the stratum; simultaneously injecting air into the stratum thru one or more boreholes spaced apart from the heated borehole; shutting in the heated borehole so that air cannot flow into it except to fill its volume at formation pressure; when ignition temperature has been reached, allowing air to flow from the stratum into the ignition borehole whereby fluids produced from the stratum pass into the ignition borehole and out production tubing positioned therein; pressurizing the ignition borehole with air injected 'fhru the annulus around the production tubing so as to force air into the heated stratum; and repeating the. cyclical passageof air into the stratum from the ignition borehole and out of the stratum into the ignition borehole at sufficiently frequent intervals that liquid in "the hot stratum is not moved sufficiently far from the-ignition borehole 'to plug the stratum and stop the flow of air thru 'the "stratum. In this manner of operation, hydrocarbons in the entire permeable zone around the ignition 'b'o're hole can be ignited prior to propagation of the combustion zone by either direct or inverse air injection technique. When utilizing this embodiment of the invention is' W61l to space the air injection boreholes a few feet such as 1 to 5 feet from the ignition borehole.

V A more complete understanding 'ofthe invention may be had by reference to the accompanying schematic drawing of which FIGURE '1 is an elevation partly insection showing one arrangement of apparatus in an earth formation for eifecting the process of the invention and FIGURE 2 is a plan view of well spacing showing a typical 'S-spot well pattern.

Referring to FIGURE 1, a carbonaceous stratum 1-0 is ipenet'rated by an ignition borehole -12 and a series of air injection boreholes 14, two of which are shown. 'Borehole 12 is 'prov-idedwit h-a heater 16 and welltubing 18 extending at least to the lower level of strata-m 1-0 and, ipreferably, to a lower level. (An air injection system comprising compressor =20, air supply line 22, and individual well feed lines 24 and 2'6 havingvalves therein, is "connected to each or the wells or boreholes thru the well heads of the respective wells. An auxiliary air injection 'line 28 connects air supply line 22 with tubing 18 at a point between valves '30 and 32. Exhaust lines 34 connectwith the wellheads of the air injection wells Heater 16 comprises any-suitable type of downhole heater such as a gas fired burner, an electric heater, and incandescent charcoal. A pressure gauge 36 is connected with the air supply system.

FIGURE 2 shows one type of well spacing-including a central ignition well and four air injection wells spaced in an annular .pattern around the ignition Well. 0bvious 1y any number of air injection wells may be utilized in the process. It is preferred to space air injection wells within about five feet of ignition well 12 in order to facilitate the prevention of the plugging of the intervening stratum; however, in strata in which plugging is not a problem, boreholes 14 may be spaced at greater distances from ignition borehole 12, such as is found in normal well spacing for in situ combustion processes.

In accordance with one embodiment of the invention heat is applied to stratumilt) by means of heater 1-6 while all of theboreholes are shut in and, when the tempera- 'ture of the strata surrounding borehole 12 has been raised to a range of about 400 to 800 F. and particularly above the ignition temperature of the carbonaceous stratum, air is injected thru line 26 by opening the valve therein while exhausting gas from boreholes 14 thru lines 34 for a short period, such as 1 to 3 minutes. At the end of this time, injection-of air thr-u line 26 is terminaed, the valves in line 34 are closed, the valves in lines 24'are open, and the valves in tubing 1-8 are both opened so as to reverse the flow of air thru the hot stratum around borehole 12. Flow of air into the ignition borehole is continued for a similar short period after which the flow is reversed by repeating the injection of air thru line 26 and exhausting -gas thru lines 34. Repeating this procedure produces sustained combustion in stratum .16 between the ignition and injection boreholes because of the high temperature of the rock and the feeding of available oxygen directly into the hot rock. Auxiliary air line '28 may be utilized for air injection in lieu of line 26 particularly when tubing 18 is free of hydrocarbon liquid.

In accordance with another embodiment of the invention, as the temperature of the stratum around borehole 12,-is raised to ignition temperature well 12 is shut in and thestratum is pressurized by injection of air 'thru boreholes 14 by proper manipulation of valves in the air supply lines. After ignition temperature has been reached, pressure in the ignition borehole is reduced by operation of valve 30 and/or valve 32 so as to permit how of air into borehole 12 and also flow of hot liquid hydrocarbon into this borehole. Pressure is again increased within borehole '12 and stratum 10 after a short period by injection of air thru line 26 so as to force air 'thru the hot stratum around borehole 12"and, after a short period, pressure is again reduced within borehole 12 by operation of valves 31 and 32 butnot belowthe pressure required'to force liquid hydrocarbon from the bottom of bore hole 21 to the well head thru tubing 18. Pressure is cyclically increased and decreased in this manner thereby forcing air into and out of the hot rock around borehole 12 so as to establish continuous combustion "therein and prevent plugging ot the formation, while removing the liquid and gaseous hydrocarbons thru tubing 18 produced in the-combustion process.

In practicing a further embodiment of the invention, well 12 is shut in by closing valve 32 and the valve in line 26 either before or after heating of the stratum around borehole 1.2 has commenced and, particularly, before the final phases of the heating prior to ignition, and the stratum is pressurized by injection of air thru boreholes 14 via lines 34-. When ignition temperature is ap'pro'achedor reached, valve-32 is partially opened (valve 30 being open and the valve in line 28 being closed) 'so as to allow air to flow thru the stratum into borehole '12. This procedure causes flow of-hot liquid hydrocarbon into borehole '12 and flow of the liquid thru tubing 15. After a short period of how. th'ru tubing 18, valve 32 1s closed and air is injected thru line 26 and is forced into stratum 10 around borehole 1-2. Flow of air line '26 is'continued for only a short period so as not to force liquid hydrocarbon away from borehole 1-2 far enough to. form a solid bank of liquid in the stratum and shut off the flow of air therethru. When the introduction of air thru line '26 is terminated, valve 32 is again partially opened so. as to allow air to again flow into the ignition borehole from the stratum and again flush liquid hydrocarbon into the borehole, while effecting efficient mlxlng of air and-fueL-in the hot stratum adjacent borehole 12.

This cyclic procedure is continued until ignition of the entire section of formation in a ring around borehole 12 has been effected. Where boreholes 14 are spaced a distance in the range of about 1 to 5 feet from borehole 12, the combustion zone established during the foregoing procedure extends from borehole 12 to boreholes 14. In an application of the invention where the injection boreholes 14, are spaced more remotely from borehole 12, the combustion zone around borehole 12 may be eX- tended any suitable distance such as one to several feet before terminating the cyclical procedure and moving the combustion zone thru the stratum outwardly from borehole 12 either by direct injection of air thru borehole 12 or by inverse injection thru injection boreholes spaced in a ring around borehole 12 at a distance of to 50 or 100 feet or more.

The foregoing procedure involves cyclic pressurizing and partial depressurizing the heated stratum around borehole 12 by injection of air thru line 26 and producing thru tubing 18. This procedure may be varied by intermittently pressurizing thru lines 34 and boreholes 14 as a substitution for injection of air thru line 20. The period of flow in either direction is to be generally limited to two or three minutes especially during the early stages of the cyclical process when the presence of hydrocarbon liquid in the stratum around borehole 12 is at a maximum. Longer periods of flow into the borehole from the stratum can be tolerated without plugging but better mixing of hydrocarbon fuel and air is obtained by maintaining the flow into the borehole relatively short before repressurizing thru line 26. The first flow of air thru the hot stratum in the various embodiments of the process should generally be into the ignition borehole so as to flush the initial liquid hydrocarbon into the borehole instead of deeper into the stratum.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

1. A process for initiating combustion in a stratum containing combustible carbonaceous material comprising heating to ignition temperature a section of said stratum around an ignition borehole therein from a heat source in said borehole; while heating said section, cyclically passing O -containing gas from an injection borehole spaced from said ignition borehole thru said section of stratum into said ignition borehole and from said ignition borehole into said section; and continuing the cyclic passing of said gas until ignition is effected.

2. The process of claim 1 wherein said O -containing gas is injected into said stratum thru a series of injection boreholes surrounding said ignition borehole during one phase of the cycle and gas is withdrawn from said ignition borehole; and during another phase of the cycle 0 containing gas is injected thru said ignition borehole and gas is withdrawn from said stratum thru said injection boreholes.

3. The process of claim 2 including the step of terminating the cycling after ignition has been established and continuously passing air thru said section of stratum in the same direction so as to move the combustion zone outwardly from said ignition borehole; and recovering produced hydrocarbons from said stratum.

4. The process of claim 3 wherein air is passed continuously thru said injection boreholes and produced hydrocarbons are recovered thru said ignition bore hole.

5. The process of claim 3 wherein air is passed con tinuously thru said ignition borehole and produced hydrocarbons are recovered thru outer boreholes.

6. A process for initiating combustion in a stratum containing carbonaceous material comprising simultaneously heating a section of said stratum around an ignition borehole from a heat source therein and alternately passing air from said ignition borehole thru said section toward an injection borehole spaced from said ignition borehole and thru said section into said ignition borehole from said injection borehole; and continuing said heating and passing of air thru said section so as to raise the temperature of same to ignition temperature and eifeet in situ combustion.

7. The process of claim 6 wherein air is injected into said stratum thru a series of injection boreholes surrounding said ignition borehole and same is passed thru said section into said borehole.

8. The process of claim 7 wherein said injection boreholes are spaced within 5 feet of said ignition borehole.

9. The process of claim 7 wherein said injection bore holes are spaced within 5 feet of said ignition borehole and continuous flow of air in either direction is limited to a maximum of about 3 minutes.

10. The process of claim 6 wherein said air is passed thru an annulus around a tubing string in said ignition borehole extending to a lower level of said stratum so as to force liquid out of the borehole thru said tubing and wherein the pressure of said air is alternately increased and decreased to efiect flow of air into and from said stratum.

11. The process of claim 6 wherein said ignition borehole is shut in during at least the later phases of heating, air is injected into said stratum thru at least one injection borehole in said stratum spaced from said ignition borehole so as to pressurize said stratum and said ignition borehole, and air pressure in said ignition borehole is cyclically decreased and increased so as to force air from and into said section and liquid hydrocarbon out thru tubing Within said ignition borehole.

12. The process of claim 11 wherein cyclical ilow of air is terminated after ignition of carbonaceous material in said section and the resulting combustion is advanced radially from said ignition borehole by continuous flow of air thru said stratum toward said ignition borehole, and produced hydrocarbons are recovered thru said ignition borehole.

References Cited in the file of this patent UNITED STATES PATENTS 2,771,954 Jenks Nov. 27, 1956 2,793,696 Morse May 28, 1957 2,818,117 Koch Dec. 31, 1957 2,862,557 Von Utcnhove Dec. 2, 1958 

1. A PROCESS FOR INITIATING COMBUSTION IN A STRATUM CONTAINING COMBUSTIBLE CARBONACEOUS MATERIAL COMPRISING HEATING TO IGNITION TEMPERATURE A SECTION OF SAID STRATUM AROUND AN IGNITION BOREHOLE THEREIN FROM A HEAT SOURCE IN SAID BOREHOLE, WHILE HEATING SAID SECTION, CYCLICALLY PASSING O2-CONTAINING GAS FROM AN INJECTION BOREHOLE SPACED FROM SAID IGNITION BOREHOLE THRU SAID SECTION OF STRATUM INTO SAID IGNITION BOREHOLE AND FROM SAID IGNITION BOREHOLE INTO SAID SECTION, AND CONTINUING THE CYCLIC PASSING OF SAID GAS UNTIL IGNITION IS EFFECTED.
 2. THE PROCESS OF CLAIM 1 WHEREIN SAID 02-CONTAINING GAS IS INJECTED INTO SAID STRATUM THRU A SERIES OF INJECTION BOREHOLES SURROUNDING SAID IGNITION BOREHOLE DURING ONE PHASE OF THE CYCLE AND GAS IS WITHDRAWN FROM SAID IGNITION BOREHOLE, AND DURING ANOTHER PHASE OF THE CYCLE O2- 